1. Field of the Invention
The present invention generally relates to an electronographic image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of these functions, and a controlling method thereof.
2. Discussion of the Background Art
Electrophotographic image forming apparatuses typically form an electrostatic latent image on a photoreceptor and develop the electrostatic latent image with a dry toner into a toner image. The toner image is then electrostatically transferred from the photoreceptor onto a recording medium such as a sheet, and fixed thereon.
However, a surface of the recording medium has a certain degree of roughness and consequently the recording medium does not completely and evenly contact the photoreceptor but forms uneven gaps therebetween. Such gaps distort a transfer electrical field and cause a repulsive Coulomb force among toner particles. Further, because electrical resistivity of the recording medium is related to a moisture content therein and depends on environmental conditions around the image forming apparatus, such as temperature and humidity, the toner image might be distorted as the environmental conditions change.
Still further, such surface roughness of the recording medium makes gloss of the fixed image uneven. In particular, uniform gloss is preferred in color images having a relatively high ratio of solid areas to clear areas (hereinafter referred to as solid portion ratio).
To solve the problems described above, one known image forming apparatus includes an intermediate transfer member facing a photoreceptor, a heating member, and a transfer fixing unit.
In such an image forming apparatus, multicolor toner images are primarily transferred from the photoreceptor, superimposed one on another on the intermediate transfer member, and fused by the heating member. Then, the transfer fixing unit secondarily transfers the fused toner image from the intermediate transfer member onto the recording medium and fixes the toner image thereon.
The method described above, in which transferring the toner image onto the recording medium and fixing the toner image thereon are simultaneously performed, is hereinafter referred to as a transfer fixing method. By contrast, a method in which fixing the toner image on the recording medium is performed separately from transferring the toner image thereonto is hereinafter referred to as simply a fixing method.
The transfer fixing method can achieve a relatively high level of gloss that is uniform and reduce image failure due to the surface roughness of the recording medium and change in the resistivity thereof.
However, in the image forming apparatus described above, the intermediate transfer member is heated for a time period during which the toner image is heated, and the photoreceptor is heated when the heated portion of the intermediate transfer member contacts the photoreceptor. In such a case, a subsequent toner image formed on the photoreceptor is likely to adhere to the photoreceptor, as a result of which physical characteristics of the photoreceptor might change.
Although cooling the intermediate transfer member with a cooler and preventing temperature rising thereof with a heat sink have been proposed so as to prevent such adhesion of the toner image to the photoreceptor, cooling the intermediate transfer member adequately is difficult. Further, such a cooler increases both the cost of the image forming apparatus as well as electrical consumption.
Another known image forming apparatus applying the transfer fixing method in which the intermediate transfer member is not heated directly is described below.
In such an image forming apparatus, four photoreceptors on which yellow, cyan, magenta, and black toner images are respectively formed are provided along a horizontally-extending transfer surface of the intermediate transfer belt, and the transfer fixing unit is located downstream of the photoreceptors in a direction in which the intermediate transfer belt moves. The transfer fixing unit includes a transfer fixing roller facing the intermediate transfer belt, a pressure roller pressing against the transfer fixing roller, and a heater to heat the transfer fixing roller.
The toner images formed on the photoreceptors are transferred and superimposed one on another on the intermediate transfer belt. This superimposed toner image is then transferred from the intermediate transfer belt onto the transfer fixing roller and fused thereon. Then, in a nip formed between the transfer fixing roller and the pressure roller, the toner image is transferred from the transfer fixing roller onto a recording medium and fixed thereon simultaneously.
In the image forming apparatus described above, because the intermediate transfer belt is not directly heated and accordingly receives a relatively small amount of heat, change in the physical characteristics of the photoreceptor and the adhesion of the toner thereto can be prevented or reduced. Further, because the toner image is fused into a film on the transfer fixing roller and then transferred therefrom onto the recording medium, a higher gloss level can be achieved.
In a color mode, images having a relatively high solid portion ratio, such as pictures and illustrations, are output and a relatively high level of uniform gloss is preferred. By contrast, in a monochrome mode, images having a relatively low solid portion ratio, such as business documents in which a ratio of lines and characters to pictures and illustrations is higher, are output.
Further, in business documents, a high level of gloss is not always preferred because relatively high gloss documents tend to reflect light, which decreases viewability. Generally, utilization of a monochrome mode and a color mode in color electronographic image forming apparatus is about 50%, respectively.